Levels of glomerular nonenzymatic glycosylation and lipid peroxide in streptozotocin (STZ)-induced diabetic rats were examined. Isolation of glomeruli was performed using a sieving technique. The levels of nonenzymatic glycosylation in the glomeruli of these rats were measured by the thiobarbituric acid (TBA) method. Measurement of lipid peroxide, i.e., malondialdehyde (MDA), levels in the renal cortex, medulla and isolated glomeruli was performed by the TBA test. Light-microscopic and immunofluorescent examinations were also performed. An increase in nonenzymatic glycosylation in the glomeruli was observed in the early phase, i.e., after 4 and 12 weeks, in STZ-induced diabetic rats. The levels of MDA in the renal cortex of 12-week-old STZ-induced diabetic rats were also significantly increased compared with those of control rats at the same age. Levels of MDA in the glomeruli of 12-week-old STZ-induced diabetic rats were slightly increased compared with those of control rats, but there was no statistical significance. In immunofluorescence, IgG or IgM was deposited in the glomerular mesangial areas and capillary walls in 12-week-old diabetic rats. However, there was no significant change in renal tissues after 4 and 12 weeks in STZ-induced diabetic rats. It was concluded that glomerular nonenzymatic glycosylation and lipid peroxide were already increased in the early phase of STZ-induced diabetic rats prior to the appearance of marked histologic alterations.